Over the past several decades the control of air pollution has become a priority concern of society. The United States and other countries have developed elaborate regulatory programs aimed at requiring factories and other major sources of air pollution to install the best available control technology (BACT) for removing contaminants from gaseous effluent streams released into the atmosphere. The standards for air pollution control are becoming increasingly stringent, so that there is a constant demand for ever more effective pollution control technologies. In addition, the operating costs of running pollution control equipment can be substantial, so there is also a constant demand for more energy efficient technologies.
One well-known type of device for removing contaminants from a gaseous effluent stream is the electrostatic precipitator (ESP). ESPs are generally recognized as being capable of high particle collection efficiency, especially of fine particles, when the particles have the proper electrical resistivity; see, e.g., O. Tassicker and J. Schwab, High-Intensity Ionizer for Improved ESP Performance, EPRI Journal (June/July 1977), pp. 56 et seq. The optimum range of dust resistivity in situ is typically between about 10.sup.8 and 10.sup.11 ohm cm. In many industrial applications, the suspended dust particles in the effluent gas streams are not in this range for the gas conditions entering the ESP. Therefore the dust particles must be conditioned prior to entry into the ESP by changing the gas temperature or increasing moisture content of the gases or both; see, e.g., G. Werner, Electrostatic precipitators in cement plants, International Cement Review (August 1991), pp. 61 et seq.; and, J. R. Riley and John M. Tate, Re-evaluating evaporative gas conditioning: Is feasibility still an issue?, International Cement Review (November 1990), pp. 36, et seq.
A typical application for an ESP is in cement manufacturing. While cement manufacturing will be described herein for illustrative purposes, it is to be understood that the present invention is equally applicable to other manufacturing processes. In the manufacture of cement, the operation of a gas conditioning tower (GCT) is critical to the successful capture of the cement dust particles by the downstream ESP. FIG. 1, described in detail below, shows a process flow diagram of the typical air pollution equipment for the cement kiln/preheater tower gases in a modern cement plant. The GCT 10 is used to condition the hot, dry gases from the preheater tower (shown schematically by arrow 15) by reducing their temperature and increasing the moisture content by injecting cooling water into gas flow. The cooling water serves both to directly lower the temperature of the flow and to increase its moisture content. Most of the time, while the mill is in normal operation, the gas flows through the raw feed mill 20 and the moist raw feed in the mill assists GCT 10 in conditioning the gases before they enter the ESP 30. The raw feed in mill 20, i.e., cool, wet limestone, creates a large heat exchange surface, decreasing the temperature of the gases and increasing the moisture content before the they flow to ESP 30. However, when raw feed mill 20 is not in operation, GCT 10 must be operated to condition the gases before the ESP 30 without the cooling and moisture release provided when the gas flows through the raw feed mill. (When raw feed mill 20 stops operating it is normally imperative to quickly divert the hot gases from flowing through the mill to avoid damage to the mill.) Thus, two main operating conditions exist for the GCT in a cement manufacturing application: one when the raw mill is on line, "mill-on," shown by arrow 50 and the other, more difficult, condition when the raw mill is off line or bypassed, "mill-off," as shown by arrow 40.
Typical emissions of particles from the cement kiln/preheater tower off-gases is shown in the graph in FIG. 2 when the gases are conditioned by the evaporative cooling of water. In FIG. 2, the temperature in degrees centigrade of the gases entering the ESP is shown on the x-axis, while the particle emissions in mg/Nm.sup.3 is shown on the y-axis. As shown by curve 210, the emissions are halved for each 10.degree. C. drop in temperature. The hot cement kiln/preheater tower gases (which may typically be around 400.degree. C.) must be cooled to around 150.degree. C. to have acceptable emissions of around 50 mg/Nm.sup.3 from the ESP. For the typical mill-on condition, the GCT is set to cool the gases to .apprxeq.250.degree. C. so there is sufficient heat left in the gases to preheat and dry the raw feed in the mill. (In this example, passing the gas flow through the raw feed mill further cools it to .apprxeq.150.degree. C., which is the proper temperature for the ESP.) On the other hand, in the mill-off condition, the GCT must be set to cool the gas flow to .apprxeq.150.degree. C. to insure that the effluent dust is properly conditioned for capture by the ESP.
State-of-the-art GCT designs utilize a plurality of two-fluid nozzles to inject and distribute the cooling water droplets into the gas flow inside the tower. These nozzles, such as the MICROMIST.TM. nozzles manufactured by the assignee of the present invention, and the SWIRL-AIR.TM. nozzles manufactured by Delavan, Inc., Lexington, Tenn., use compressed air to atomize the cooling water into fine droplets that quickly evaporate. A two-fluid nozzle is connected to a source of water and a source of compressed air. Both the water and compressed air supply can be controlled, and by adjusting the two flows the amount of water injected into the gas flow and the size of the droplets in the spray formed by the nozzle can be independently adjusted. There is substantial interaction between water and air pressure settings in most two-fluid nozzles, especially internal mix types. Therefore, precise control of nozzle performance under varying gas conditions is required to maintain proper ESP performance at all times. As might be expected, the quantity of water injected by the nozzle is largely dependent on the pressure of water flowing to the nozzle. The size of the droplets formed by the nozzle, on the other hand, is more dependent on the compressed air flow to the nozzle. While finer droplets evaporate and exchange heat faster than larger droplets, they require a greater quantity of compressed air. Supplying the additional compressed air, in turn, requires a greater energy input and, thereby, reduces the operational efficiency of the air pollution control system.
Under normal operating conditions, the water spray into the GCT should, preferably, be totally evaporated before reaching the bottom of the tower. Normally, water flow to the nozzles is modulated to control the temperature of the gases exiting the GCT to a desired set point, or target temperature, by controlling the quantity of water injected into the GCT. Sometimes the compressed air pressure and, therefore, air flow to the nozzles is also modulated to help maintain a temperature set point by varying the droplet size.
A particularly difficult requirement for an ESP used in the above example is to maintain high collection efficiency when the raw mill unexpectedly goes off-line. Hot, dry gases from the GCT at .apprxeq.250.degree. C. for the mill-on condition are immediately routed to the ESP to protect the raw mill from damage. These hot gases immediately degrade ESP performance as indicated in FIG. 2. The outlet temperature set point of the GCT must be reset to the mill-off value, i.e., .apprxeq.150.degree. C., for the new mill-off condition. In the prior art, the temperature set point is reset manually and the control system for maintaining the new temperature set point for the GCT relies on a temperature sensor at the output of the GCT which continuously measures the temperature of the exiting gases and adjusts the nozzles via a controller to maintain a desired temperature set-point. Examples of prior art control techniques are disclosed in U.S. Pat. No. 3,842,615, and in a brochure entitled "MicroMist.TM. Evaporative Gas Cooling and Conditioning Systems"distributed by the assignee of the present invention, the disclosure of which is incorporated by reference.
However, in prior art methods, by the time the temperature set point is physically changed and the temperature sensors, nozzles and controllers respond to bring the system to the new temperature set point, the "filter cake" on the collecting plates in the ESP becomes dried out and highly resistive. It then takes much more time to recondition the filter cake layers than it did to heat them up and dry them out. Typically the filter cake layers dry out very quickly, sometimes in less than a minute, while it may take 30 minutes to 2 hours after proper gas conditioning has been restored to the ESP, before the ESP returns to normal operation. This type of event often results in emissions by the plant which exceed allowable levels which may constitute a violation of applicable pollution control standards.
The costs of operating a pollution control system can be very significant. Energy costs are often the paramount factor in determining the overall operation expense of the system. In GCTs of the type which utilize two-fluid nozzles, much of the energy consumed by the system is used to create the compressed air employed by the system. Many prior art systems, of the type heretofore described, are designed such that the GCT is optimized to efficiently cool the gas flow during upset conditions, i.e., when the demands on the system are the greatest. The prior art systems are not optimally efficient under normal operating conditions, as when the feed mill is on line, and the cooling requirements are reduced.
Accordingly, it is an object of the present invention to provide an improved automatic control system for evaporative cooling which responds immediately to the needs of the downstream collector.
Another object of the present invention is to provide an improved automatic control system for evaporative cooling wherein the size of the injected droplets are controlled for the conditions of use.
Another object of the present invention is to provide an improved automatic control system for evaporative cooling which minimizes the compressed air consumption of the atomizing nozzles according to the demands of the cooling system.
Yet another object of the present invention is to provide an improved automatic control system for evaporative cooling which maintains conditions that result in a high collection efficiency in the downstream collector while reducing the energy input to the system, as compared to the prior art.